vswitchd: Document map members as separate columns
The OVS configuration database now has numerous columns that contain fixed
key-value pairs. Currently there's no way to see these at a glance,
because they are not presented in the summary tables just before the
detailed descriptions.
This commit extends the XML format so that keys within a column can be
described individually, and rearranges and rewrites vswitch.xml to take
advantage of this feature.